This invention relates to a ground monitoring system and more particularly to a system for monitoring the integrity of the ground wire in a multi-wire power cable of a type used in surface mining operations, i.e., strip mines. In surface mining operations, extensive use is made of large electrically powered machinery such as power shovels, pumps, drills, etc. As an example, 600 volt to 25 kilovolt, three phase electrical service may be provided at a remote surface mining site. Electrical utility company power lines are brought to a substation at or close to the mining site. Because much of the mining machinery must be movable, long insulated power cables that can be dragged along the ground connect the machinery to mobile switch houses that in turn are connected to the substation by additional long insulated cables. There are a number of different types of power cables used in surface mining. One such type of cable includes the three power conductors or phase wires, each with a metallic shield around it, two bare ground conductors, and an insulated ground check wire that commonly is called a pilot wire. All are enclosed in an insulating elastomeric jacket. Typically, the cables are in 1,000 or 1,500 foot lengths. Two or more cables, and sometimes up to ten cables are series connected.
If an insulation breakdown occurs in one of the large electrically powered pieces of machinery the entire machine may be at a dangerous high voltage unless it is protected by a suitable metallic ground circuit. In many instances the machine will be in contact with the earth, but because the machinery is moved around from place to place, a dependable earth ground resulting only from contact with the earth cannot be relied upon. Consequently, a metallic ground circuit in the power cable is essential. The Federal Coal Mine Health and Safety Act of 1969 requires that the electrical ground circuit in the power cable be continuously monitored to detect short circuits, open circuits, and abnormally high resistance conditions in the ground circuit. Upon detection of a fault condition the ground monitor must open one or more circuit breakers to disconnect the electrical source from the faulty portion of the system.
A ground monitor intended for use at a surface mining location must be able to detect the desired condition that it is the metallic ground conductor of the power cable that is providing ground continuity in the system and not an earth ground resulting from the machine being in contact with the earth or water.
A system for monitoring the integrity, or continuity, of the ground conductor of an electrical power cable is disclosed in my U.S. Pat. No. 4,228,475, issued Oct. 14, 1980. In that system, corresponding ends of the pilot wire and ground conductor of a power cable are connected together through a terminating resistor Rt. The other ends of the ground conductor and pilot wire are connected to the primary ground of the mining site. A 1 kHz sensing signal is coupled onto the loop formed by the pilot and ground conductors. The ground sensing circuit of that patent continually monitors the loop and produces a fault indication when the resistance "seen" by the sensing circuit increases by more than two ohms from the value of the terminating resistance Rt. An objective in that system was to assure that the sensing signal in the loop was not diverted through any sneak paths to earth ground at a switch house, splice skids, and machinery frames, for example. To isolate the pilot wire and ground conductor loop from potential sneak paths, a large blocking impedance in the form of a large iron core inductor was connected in series with the ground conductor just beyond the junction with the terminating resistor Rt. In U.S. Pat. No. 4,321,643, issued Mar. 23, 1982 to R. Vernier, the blocking impedance is located in the line that connects to the local ground stake rather than in series with the cable ground conductor or conductors.
The ground conductor monitoring system disclosed in my above mentioned patent performs well to achieve its desired purpose, but does have several disadvantages. The blocking impedance is quite expensive, is difficult to install on equipment in the field, and is not fail-safe because it appears as a high impedance to the sensing signal whether it is functioning as intended or is faulty because it has an open circuit therein.
Another shortcoming of the system disclosed in my above mentioned patent results from the fact that the fault sensing circuitry is "looking for" a resistance change of two ohms in the loop formed by the pilot wire and ground conductor. In order for a two ohm change in resistance to appear to be significant and thus easily detected, the total resistance in the loop must be a low value. This means that the impedance in the ground sensing circuitry that is in series with the loop has to be as small as possible. This consideration severely restricts the use of noise suppressing and transient suppressing circuit means in the fault sensing circuitry. Because of the restriction on the use of noise suppressing means, the fault sensing circuitry of my previous system was to some extent susceptible to damage and destruction of components due to noise and transients.